This invention relates to an ignition coil unit for an internal combustion engine and, more particularly, to an ignition coil unit in which an ignition coil and a power switch for controlling a primary current through the ignition coil are integrally combined into a unit.
FIG. 3 is an electrical circuit diagram of a known ignition coil unit for an internal combustion engine. The ignition coil unit comprises an ignition coil A having a primary coil 2 and a secondary coil 7, and a power switch circuit B having a plurality of electric and electronic circuit components. In FIG. 3, it is also seen that an electric source C and an ignition signal control circuit D are connected to the ignition coil unit.
The power switch circuit B comprises a power transistor 1 for switching a primary current flowing through the primary coil 2 of the ignition coil A, a current limiting circuit 4 and a current detecting circuit 3 for detecting a potential difference generated by the primary current and for transmitting a primary current control signal to the current limiting circuit 4.
FIG. 4 is a sectional side view of a conventional ignition coil unit in which the ignition coil A and the power switch circuit B are integrally combined. In FIG. 4, reference numeral 5 designates a casing, 6 is a cylindrical open-circuit magnetic iron core concentrically disposed within the casing 5, 7 is a secondary coil wound on the outer circumference of a primary coil 2 which is wound around the open-circuit magnetic iron core 6, 8 is an aluminum heat dissipating plate mounted on the longitudinal axis of the open-circuit magnetic iron core 6, and 9 is a resin-molded IC package bonded to the heat dissipating plate 8. The IC package 9 is composed of the control circuit B including the power transistor 1, the current limiting circuit 4, etc. transfer-molded with a suitable resin. Reference numeral 10 is an outer magnetic iron core mounted to the inner circumferential surface of the casing 5, and 11 is an electrical connector.
In the conventional ignition coil unit as above described, the primary current of the primary coil 2 flows through the current detection circuit 3, where the current level is detected as the potential difference upon which a control signal is supplied to the current limiting circuit 4. The current limiting circuit 4 controls, in accordance with this control signal, the primary current flowing through the primary coil 2 of the ignition coil A. In response to this primary current flowing through the primary coil 2, a high voltage to be supplied to a distributor (not shown) is generated in the secondary coil 7 of the ignition coil A.
With the conventional ignition coil unit as above described, the magnetic iron cores 6 and 10 of the ignition coil A are arranged such that the magnetic flux generated by the primary and the secondary coils 2 and 7 flows between two magnetic iron cores 6 and 10 at their axial ends. Therefore, some portion of the magnetic flux emitted from the magnetic iron cores 6 and 10 reaches the resin-molded IC package 9 as a leakage magnetic flux. When this leakage magnetic flux passes through the power switching element 1 within the IC package 9, an electric current induced by the leakage flux causes an output signal from the power switching element 1 to deviate from its correct value, resulting in an erroneous operation of the power switch circuit B.